27.4.Continuous data logging
The purpose of the continuous data log is to log data periodically without interruption. This type of data log is useful mainly for persistent processes.
With continuous data logging, you define the log rate in milliseconds. In this period, all data defined in a data log are read from PLCs and logged into the internal database.
For any data point, you can define hysteresis. When hysteresis is enabled, mySCADA will read values each Read cycle, defined by the Read Refresh parameter; however, it will log data only if the value has changed by more than the specified Delta Abs value.
When hysteresis is enabled for at least one data point, you should specify read refresh and log refresh.
Read refresh: how often data will be read from the PLC
Log refresh: how often data will be logged when there is no change in value (e.g. no log triggered by value change > delta abs).
Hysteresis function can be especially useful for logging analog data that does not change very often. When you set hysteresis of some data points in the data log, you can specify the log rate – the period in which all data points of the data log will be logged, regardless whether they have changed or not.
By enabling hysteresis, you can dramatically decrease the amount of historical data logged while maintaining data precision and time continuity.